A speech encoding technology that compresses speech signals at a low bit rate is important to use radio waves etc. efficiently in a mobile communication system. Further, in recent years, expectations for improvement of quality of communication speech have been increased, and it is desired to implement communication services with high realistic sensation. It is therefore not only desirable for a speech signal to become higher in quality, but also desirable to code signals other than speech such as an audio signal with a broader bandwidth with high quality.
An encoding technology is therefore required that is capable of achieving high quality when a radio wave reception environment is good, and achieving a low bit rate when the reception environment is inferior. In response to this requirement, an approach of hierarchically incorporating a number of encoding technologies to provide scalability shows promise. Scalability (or a scalable function) indicates a function capable of generating a decoded signal even from a part of an encoded code.
FIG. 1 is a block diagram showing a configuration for two-layer hierarchical encoding apparatus 10 as an example of hierarchical encoding (embedded encoding, scaleable encoding) apparatus of the related art.
Sound data is inputted as an input signal, and a signal with a low sampling rate is generated at down-sampling section 11. The down-sampled signal is then given to first layer encoding section 12, and this signal is coded. An encoded code of first layer encoding section 12 is given to multiplexer 17 and first layer decoding section 13. First layer decoding section 13 generates a first layer decoded signal based on the encoded code. Next, up-sampling section 14 increases the sampling rate of the decoded signal outputted from first layer decoding section 13. Delay section 15 then gives a delay of a predetermined time to an input signal. The first layer decoded signal outputted from up-sampling section 14 is then subtracted from the input signal outputted from delay section 15 to generate a residual signal, and this residual signal is given to second layer encoding section 16. Second layer encoding section 16 then codes the given residual signal and outputs an encoded code to multiplexer 17. Multiplexer 17 then multiplexes the first layer encoded code and the second layer encoded code to output as an encoded code.
Hierarchical encoding apparatus 10 is provided with delay section 15 that gives a delay of a predetermined time to an input signal. This delay section 15 corrects a time lag (phase difference) between the input signal and the first layer decoded signal. The phase difference corrected at delay section 15 occurs in filtering processing at down-sampling section 11 or up-sampling section 14, or in signal processing at first layer encoding section 12 or first layer decoding section 13. A preset predetermined fixed value (fixed sample number) is used as a delay amount for correcting this phase difference, that is, a delay amount used at delay section 15 (for example, refer to Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Application Laid-open No. HEI8-46517.
Patent Document 1: Japanese Patent Application Laid-open No. HEI8-263096.